The Collaborative International Dictionary of English v.0.48:

Offhand \Off"hand`\, a.
   Instant; unprepared; ready; extemporaneous; unrehearsed; as,
   an offhand speech; offhand excuses; an offhand comment. --
   adv. In an offhand manner; as, he replied offhand.
   [1913 Webster]

WordNet (r) 3.0 (2006):

off-hand
    adv 1: without preparation; "I don't know the figures off-hand"
           [syn: off-hand, ex tempore]

Moby Thesaurus II by Grady Ward, 1.0:

161 Moby Thesaurus words for "offhand":
   Bohemian, abrupt, ad lib, ad libitum, ad-lib, affable, airily,
   airy, aloof, any old way, any which way, anyhow, at short notice,
   at sight, blase, brusque, bunglingly, by ear, by the by,
   by the way, careless, carelessly, casual, casually, cavalier,
   clumsily, cool, cordial, cursorily, cursory, curt, degage,
   devil-may-care, disregardant, disregardful, disregardfully,
   distant, easy, easygoing, en famille, en passant, extemporaneous,
   extemporaneously, extemporarily, extemporary, extempore, familiar,
   familiarly, flippant, folksy, forgetful, forgetfully,
   free and easy, glib, gracious, haphazardly, haymish, heedless,
   heedlessly, helter-skelter, hit and miss, hit or miss, homely,
   homey, impromptu, improvisatorial, improvise, improvised,
   improviso, in passing, inattentively, incidentally, inconsiderate,
   inconsiderately, indifferent, informal, informally, insouciant,
   irregular, jury-rigged, lackadaisical, lazy, light-hearted, loose,
   makeshift, messily, natural, naturally, nonchalant, oblivious,
   off the cuff, off the hip, off-the-cuff, offhanded, offhandedly,
   on sight, once over lightly, out of hand, parenthetically,
   perfunctorily, perfunctory, plain, plainly, promiscuously,
   reckless, recklessly, regardless, regardlessly, relaxed, relaxedly,
   respectless, sans ceremonie, simple, simply, slapdash, sloppily,
   smooth, sociable, spur-of-the-moment, stopgap, superficial,
   superficially, tactless, tactlessly, thoughtless, thoughtlessly,
   unaffected, unaffectedly, unassuming, unassumingly, unceremonious,
   unceremoniously, unconcerned, unconstrained, unconstrainedly,
   unconventional, undiplomatic, ungracious, unguardedly, unheedful,
   unheedfully, unheeding, unheedingly, uninterested, unmindful,
   unmindfully, unofficial, unofficially, unpremeditated, unprepared,
   unready, unrehearsed, unsolicitous, unsolicitously, unstudied,
   untactful, unthinking, unthinkingly, unvigilantly, unwarily,
   without ceremony